Flexure based guidance system for varifocal head-mounted displays

ABSTRACT

A varifocal head mounted display (HMD) includes an electronic display, an optical system, and the guidance system. The electronic display presents content. The optical system includes one or more optical elements and provides the content to an eyebox of the HMD. The guidance system is a flexure based guidance system that includes an actuator and a first and second flexure elements (e.g., parallel beam, dual Roberts, etc.) guiding movement of the electronic display along an optical axis of the optical system in order to adjust a location of one or moveable elements in the optical system and, thereby, control a location of an image plane. The first and second flexure elements are able to flex or bend with movement of the actuator to adjust the location of the one or moveable elements that includes the electronic display and/or one or more optical elements of the optical system.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Application No. 62/685,396,filed Jun. 15, 2018, which is incorporated by reference in its entirety.

BACKGROUND

This disclosure relates generally to guidance systems, and in particularto flexure based guidance systems for varifocal head-mounted displays.

A head mounted display (HMD) can be used to provide virtual images to auser. For example, stereoscopic images are displayed on an electronicdisplay inside the HMD to simulate the illusion of depth and headtracking sensors estimate what portion of the virtual environment isbeing viewed by the user. Conventional HMDs are often unable tocompensate for vergence and accommodation conflicts when renderingcontent, which may cause visual fatigue and nausea in users.

SUMMARY

A varifocal head mounted display (HMD) includes an electronic display,an optical system (e.g., a pancake lens assembly), and the guidancesystem. The electronic display presents content. The optical systemincludes one or more optical elements and provides the content to aneyebox of the HMD. The guidance system is a flexure based guidancesystem that includes an actuator and a system of one or more flexureelements guiding movement of the electronic display along an opticalaxis of the optical system in order to adjust a location of one or moremoveable elements in the optical system and, thereby, control a locationof an image plane. The system of one or more flexure elements are ableto flex or bend with movement of the actuator to adjust the location ofthe one or more moveable elements that includes the electronic displayand/or one or more optical elements of the optical system.

The guidance system, in one embodiment, is a parallel beam flexureguidance system that includes an actuator (e.g., a voice coil actuator,stepper motor, brushless DC motor, etc.), a first pair of flexure beamsconfigured to guide movement of the electronic display 102 along anoptical axis of optical system and to prevent tip/tilt in a firstdimension (i.e., x-axis), and a second pair of flexure beams configuredto guide movement of the electronic display 102 along an optical axis ofoptical system and to prevent tip/tilt in a second dimension (i.e.,y-axis). In this embodiment, the actuator is positioned off-axisrelative to the optical axis and the first and second pair of flexurebeams are arched in shape to conform to a shape around the opticalsystem to minimize the form factor of the HMD. At one end, the actuatoris fixed relative to the electronic display in order to move electronicdisplay and, at the other end, the actuator is fixed relative to theoptical system. Accordingly, the first and second pairs of flexure beamsextend from the actuator while curving around the optical system tolocations that are fixed relative to the optical system. Thus, whenactuator is operated, the end of the actuator fixed relative to theelectronic display is extended to move the electronic display relativeto the optical system and, thereby, adjusting the location of the imageplane while the first and second pairs of flexure beams are flexed orbent away from the optical system.

In another embodiment, the guidance system is a Roberts flexure guidancesystem that includes an actuator, three flexure elements, a pivot pointwhere each of these flexure elements are connected in a triangulararray, a first anchor for the central flexure element of this triangulararray that is fixed relative to the optical system, and a pair of secondanchors that each connect to one of the outer flexures of the triangulararray that are fixed relative to the electronic display and the movingend of the actuator. Thus, when actuator is operated, the end of theactuator fixed relative to the electronic display is extended to movethe electronic display relative to the optical system to adjust thelocation of the image plane while the flexure elements are flexed aboutthe pivot point. This causes the pair of second anchors to move in thez-direction relative to the first anchor, thereby, producing linearmotion along the optical axis in a predictable and repeatable manner.

Accordingly, these flexure guidance systems produce linear motion alongan optical axis of the optical system while minimizing parasitic errormotions including straightness of travel of electronic display relativeto the optical axis while minimizing angular error. Moreover, theseflexure guidance systems produce this motion within a relatively smallform factor while minimizing power requirements.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows an example virtual reality system, in accordance with atleast one embodiment.

FIG. 2 shows a diagram of a parallel beam flexure guidance system foruse within a head mounted display, in accordance with at least oneembodiment.

FIG. 3 shows another diagram of a parallel beam flexure guidance systemthat includes the optics block of the head mounted display, inaccordance with at least one embodiment.

FIG. 4 shows a side view of diagram of the parallel beam flexureguidance system and optics block of FIG. 3.

FIGS. 5A-5B show a parallel beam flexure guidance system of a headmounted display in operation, in accordance with various embodiments.

FIG. 6 shows a diagram of a compound parallel beam flexure guidancesystem, in accordance with at least one embodiment.

FIG. 7 shows a diagram of a Roberts flexure guidance system for usewithin a head mounted display, in accordance with at least oneembodiment.

FIGS. 8A-8B show a Roberts flexure guidance system of a head mounteddisplay in operation, in accordance with various embodiments.

The figures depict embodiments of the present disclosure for purposes ofillustration only. One skilled in the art will readily recognize fromthe following description that alternative embodiments of the structuresand methods illustrated herein may be employed without departing fromthe principles, or benefits touted, of the disclosure described herein.

DETAILED DESCRIPTION Flexure Guidance System Overview

A varifocal system provides dynamic adjustment of the focal distance ofa head mounted display (HMD) or other near-eye optic systems to keep auser's eyes in a zone of comfort as vergence and accommodation change.The system uses an eye tracker to determine a vergence depthcorresponding to where the user is looking and adjusts the focus toensure image is in focus at the determined focal plane. The system, inone implementation, physically changes the distance between anelectronic display and one or more lenses of an optical block of the HMDby moving the electronic display, one or more lenses of the opticalblock, or a combination of both using various actuation devices.

The movement of these elements is guided by a flexure guidance system. Aflexure guidance system makes use of compliant flexure elements thatdeform/deflect deterministically when mechanically stressed. A flexureguidance system is friction-free when compared to shaft in bushingguidance, can potentially be implemented with fewer parts, and may beintegrated to molded plastic parts.

There are three main objectives in designing a flexure guidance systemfor a varifocal HMD. The first is to strive to produce pure linearmotion along the desired axis of travel while minimizing parasitic errormotions. This includes straightness of travel relative to the desiredmotion axis as well as angular error; both of which can weaken the userexperience. The desired axis of travel is along the optical axis of theoptics of the HMD. The second objective is to produce this motion withina form factor that is not cumbersome to user. The third object is tominimize power requirements since the motor must also overcome thedeformation of the flexures that guide motion. All objectives are deeplycoupled and designing for just one or two can result in unsatisfactoryperformance in the others.

To approach pure linear motion while minimizing motor requirements, insome embodiments, off-axis stiffness is maximized while the on-axisstiffness in the direction of motion is minimized. Off-axis stiffness ofa leaf flexure is highly sensitive to the length L, and width b, of theflexures (inversely proportional to L³ and proportional to b³) and thespacing between them. On-axis stiffness is highly sensitive to thethickness t, and length of the flexures (proportional to t³ andinversely proportional to L³). By reducing the thickness of individualflexures and increasing the quantity of flexures (from 2 to quantity n)to produce an array of flexures, the structural sensitivities areexploited to significantly reduce on-axis stiffness (on-axis stiffnessis proportional to number of flexures, n) with lesser effects onoff-axis stiffness.

The significant reduction in on-axis stiffness while maintaining mostoff-axis stiffness opens the solution space for flexure geometry/formfactor as well as material selection (e.g. stainless steel vs titanium).

System Overview

FIG. 1 shows varifocal system 100 in which a head-mounted display (HMD)101 operates. Varifocal system 100 may be for use as a virtual reality(VR) system, an augmented reality (AR) system, a mixed reality (MR)system, or some combination thereof. In this example, the varifocalsystem 100 includes HMD 101, imaging device 160, and I/O interface 170,which are each coupled to console 150. While FIG. 1 shows a single HMD101, a single imaging device 160, and a single I/O interface 170, inother embodiments, any number of these components may be included in thesystem. For example, there may be multiple HMDs each having anassociated I/O interface 170 and being monitored by one or more imagingdevices 160, with each HMD 101, I/O interface 170, and imaging devices160 communicating with the console 150. In alternative configurations,different and/or additional components may also be included in thesystem environment.

HMD 101 presents content to a user. Example content includes images,video, audio, or some combination thereof. Audio content may bepresented via a separate device (e.g., speakers and/or headphones)external to HMD 101 that receives audio information from HMD 101,console 150, or both. HMD 101 includes electronic display 102, opticsblock 104, varifocal actuation block 106, focus prediction module 108,eye tracking module 110, vergence processing module 112, one or morelocators 114, inertial measurement unit (IMU) 116, head tracking sensors118, and scene rendering module 120.

Optics block 104 directs light from electronic display 102 to an exitpupil for viewing by a user using one or more optical elements, such asapertures, Fresnel lenses, convex lenses, concave lenses, filters, andso forth, and may include combinations of different optical elements. Insome embodiments, one or more optical elements in optics block 104 mayhave one or more coatings, such as anti-reflective coatings.Magnification of the image light by optics block 104 allows electronicdisplay 102 to be physically smaller, weigh less, and consume less powerthan larger displays. Additionally, magnification of the image light mayincrease a field of view of the displayed content. For example, thefield of view of the displayed content is such that the displayedcontent is presented using almost all (e.g., 150 degrees diagonal), andin some cases all, of the user's field of view. The optics block 104 canbe a single lens or a system of lenses, such as a pancake lens.Additional detail regarding a pancake lens assembly is describe indetail in U.S. application Ser. Nos. 15/993,316, and 15/292,108, whichare hereby incorporated by reference in their entirety.

Optics block 104 may be designed to correct one or more optical errors.Examples of optical errors include: barrel distortion, pincushiondistortion, longitudinal chromatic aberration, transverse chromaticaberration, spherical aberration, chromatic aberration, field curvature,astigmatism, and so forth. In some embodiments, content provided toelectronic display 102 for display is pre-distorted, and optics block104 corrects the distortion when it receives image light from electronicdisplay 102 generated based on the content.

Varifocal actuation block 106 includes a varifocal actuation block thatcauses optics block 104 to vary the focal distance of HMD 101 to keep auser's eyes in a zone of comfort as vergence and accommodation change.In one embodiment, varifocal actuation block 106 physically changes thedistance between electronic display 102 and optical block 104 by movingelectronic display 102 or optical block 104 (or both), as will beexplained further with respect to FIGS. 4A-21B. Additionally, moving ortranslating two lenses relative to each other may also be used to changethe focal distance of HMD 101. Thus, varifocal actuation block 106 mayinclude actuators or motors that move electronic display 102 and/oroptical block 104 to change the distance between them. Varifocalactuation block 106 may be separate from or integrated into optics block104 in various embodiments.

Each state of optics block 104 corresponds to a focal distance of HMD101 or to a combination of the focal distance and eye position relativeto optics block 104 (as discussed further below). In operation, opticsblock 104 may move in a range of ˜5-10 mm with a positional accuracy of˜5-10 μm for a granularity of around 1000 focal distances, correspondingto 1000 states of optics block 104. Any number of states could beprovided; however, a limited number of states accommodate thesensitivity of the human eye, allowing some embodiments to include fewerfocal distances. For example, a first state corresponds to a focaldistance of a theoretical infinity meters (0 diopter), a second statecorresponds to a focal distance of 2.0 meters (0.5 diopter), a thirdstate corresponds to a focal distance of 1.0 meters (1 diopter), afourth state corresponds to a focal distance of 0.5 meters (1 diopter),a fifth state corresponds to a focal distance of 0.333 meters (3diopter), and a sixth state corresponds to a focal distance of 0.250meters (4 diopter). Varifocal actuation block 106, thus, sets andchanges the state of optics block 104 to achieve a desired focaldistance.

Focus prediction module 108 is an encoder including logic that tracksthe position or state of optics block 104 to predict to one or morefuture states or locations of optics block 104. For example, focusprediction module 108 accumulates historical information correspondingto previous states of optics block 104 and predicts a future state ofoptics block 104 based on the previous states. Because rendering of avirtual scene by HMD 101 is adjusted based on the state of optics block104, the predicted state allows scene rendering module 120, furtherdescribed below, to determine an adjustment to apply to the virtualscene for a particular frame. Accordingly, focus prediction module 108communicates information describing a predicted state of optics block104 for a frame to scene rendering module 120. Adjustments for thedifferent states of optics block 104 performed by scene rendering module120 are further described below.

Eye tracking module 110 tracks an eye position and eye movement of auser of HMD 101. A camera or other optical sensor inside HMD 101captures image information of a user's eyes, and eye tracking module 110uses the captured information to determine interpupillary distance,interocular distance, a three-dimensional (3D) position of each eyerelative to HMD 101 (e.g., for distortion adjustment purposes),including a magnitude of torsion and rotation (i.e., roll, pitch, andyaw) and gaze directions for each eye. In one example, infrared light isemitted within HMD 101 and reflected from each eye. The reflected lightis received or detected by the camera and analyzed to extract eyerotation from changes in the infrared light reflected by each eye. Manymethods for tracking the eyes of a user can be used by eye trackingmodule 110. Accordingly, eye tracking module 110 may track up to sixdegrees of freedom of each eye (i.e., 3D position, roll, pitch, and yaw)and at least a subset of the tracked quantities may be combined from twoeyes of a user to estimate a gaze point (i.e., a 3D location or positionin the virtual scene where the user is looking). For example, eyetracking module 110 integrates information from past measurements,measurements identifying a position of a user's head, and 3D informationdescribing a scene presented by electronic display element 102. Thus,information for the position and orientation of the user's eyes is usedto determine the gaze point in a virtual scene presented by HMD 101where the user is looking.

Further, distance between a pupil and optics block 104 changes as theeye moves to look in different directions. The varying distance betweenpupil and optics block 104 as viewing direction changes is referred toas “pupil swim” and contributes to distortion perceived by the user as aresult of light focusing in different locations as the distance betweenpupil and optics block 104. Accordingly, measuring distortion atdifferent eye positions and pupil distances relative to optics block 104and generating distortion corrections for different positions anddistances allows mitigation of distortion caused by “pupil swim” bytracking the 3D position of a user's eyes and applying a distortioncorrection corresponding to the 3D position of each of the user's eye ata given point in time. Thus, knowing the 3D position of each of a user'seyes allows for the mitigation of distortion caused by changes in thedistance between the pupil of the eye and optics block 104 by applying adistortion correction for each 3D eye position.

Vergence processing module 112 determines a vergence depth of a user'sgaze based on the gaze point or an estimated intersection of the gazelines determined by eye tracking module 110. Vergence is thesimultaneous movement or rotation of both eyes in opposite directions tomaintain single binocular vision, which is naturally and automaticallyperformed by the human eye. Thus, a location where a user's eyes areverged is where the user is looking and is also typically the locationwhere the user's eyes are focused. For example, vergence processingmodule 112 triangulates the gaze lines to estimate a distance or depthfrom the user associated with intersection of the gaze lines. The depthassociated with intersection of the gaze lines can then be used as anapproximation for the accommodation distance, which identifies adistance from the user where the user's eyes are directed. Thus, thevergence distance allows determination of a location where the user'seyes should be focused and a depth from the user's eyes at which theeyes are focused, thereby, providing information, such as an object orplane of focus, for rendering adjustments to the virtual scene.

In some embodiments, rather than provide accommodation for the eye at adetermined vergence depth, accommodation may be directly determined by awavefront sensor, such as a Shack-Hartmann wavefront sensor; hence, astate of optics block 104 may be a function of the vergence oraccommodation depth and the 3D position of each eye, so optics block 104brings objects in a scene presented by electronic display element 102into focus for a user viewing the scene. Further, vergence andaccommodation information may be combined to focus optics block 104 andto render synthetic depth of field blur.

Locators 114 are objects located in specific positions on HMD 101relative to one another and relative to a specific reference point onHMD 101. Locator 114 may be a light emitting diode (LED), a corner cubereflector, a reflective marker, a type of light source that contrastswith an environment in which HMD 101 operates, or some combinationthereof. Active locators 114 (i.e., an LED or other type of lightemitting device) may emit light in the visible band (˜380 nm to 750 nm),in the infrared (IR) band (˜750 nm to 1 mm), in the ultraviolet band (10nm to 380 nm), some other portion of the electromagnetic spectrum, orsome combination thereof.

Locators 114 can be located beneath an outer surface of HMD 101, whichis transparent to the wavelengths of light emitted or reflected bylocators 114 or is thin enough not to substantially attenuate thewavelengths of light emitted or reflected by locators 114. Further, theouter surface or other portions of HMD 101 can be opaque in the visibleband of wavelengths of light. Thus, locators 114 may emit light in theIR band while under an outer surface of HMD 101 that is transparent inthe IR band but opaque in the visible band.

IMU 116 is an electronic device that generates fast calibration databased on measurement signals received from one or more of head trackingsensors 118, which generate one or more measurement signals in responseto motion of HMD 101. Examples of head tracking sensors 118 includeaccelerometers, gyroscopes, magnetometers, other sensors suitable fordetecting motion, correcting error associated with IMU 116, or somecombination thereof. Head tracking sensors 118 may be located externalto IMU 116, internal to IMU 116, or some combination thereof.

Based on the measurement signals from head tracking sensors 118, IMU 116generates fast calibration data indicating an estimated position of HMD101 relative to an initial position of HMD 101. For example, headtracking sensors 118 include multiple accelerometers to measuretranslational motion (forward/back, up/down, left/right) and multiplegyroscopes to measure rotational motion (e.g., pitch, yaw, and roll).IMU 116 can, for example, rapidly sample the measurement signals andcalculate the estimated position of HMD 101 from the sampled data. Forexample, IMU 116 integrates measurement signals received from theaccelerometers over time to estimate a velocity vector and integratesthe velocity vector over time to determine an estimated position of areference point on HMD 101. The reference point is a point that may beused to describe the position of HMD 101. While the reference point maygenerally be defined as a point in space, in various embodiments,reference point is defined as a point within HMD 101 (e.g., a center ofthe IMU 130). Alternatively, IMU 116 provides the sampled measurementsignals to console 150, which determines the fast calibration data.

IMU 116 can additionally receive one or more calibration parameters fromconsole 150. As further discussed below, the one or more calibrationparameters are used to maintain tracking of HMD 101. Based on a receivedcalibration parameter, IMU 116 may adjust one or more IMU parameters(e.g., sample rate). In some embodiments, certain calibration parameterscause IMU 116 to update an initial position of the reference point tocorrespond to a next calibrated position of the reference point.Updating the initial position of the reference point as the nextcalibrated position of the reference point helps reduce accumulatederror associated with determining the estimated position. Theaccumulated error, also referred to as drift error, causes the estimatedposition of the reference point to “drift” away from the actual positionof the reference point over time.

Scene render module 120 receives content for the virtual scene fromengine 156 and provides the content for display on electronic display102. Additionally, scene render module 120 can adjust the content basedon information from focus prediction module 108, vergence processingmodule 112, IMU 116, and head tracking sensors 118. For example, uponreceiving the content from engine 156, scene render module 120 adjuststhe content based on the predicted state (i.e., eye position and focaldistance) of optics block 104 received from focus prediction module 108by adding a correction or pre-distortion into rendering of the virtualscene to compensate or correct for the distortion caused by thepredicted state of optics block 104. Scene render module 120 may alsoadd depth of field blur based on the user's gaze, vergence depth (oraccommodation depth) received from vergence processing module 112, ormeasured properties of the user's eye (e.g., 3D position of the eye,etc.). Additionally, scene render module 120 determines a portion of thecontent to be displayed on electronic display 102 based on one or moreof tracking module 154, head tracking sensors 118, or IMU 116, asdescribed further below.

Imaging device 160 generates slow calibration data in accordance withcalibration parameters received from console 150. Slow calibration dataincludes one or more images showing observed positions of locators 114that are detectable by imaging device 160. Imaging device 160 mayinclude one or more cameras, one or more video cameras, other devicescapable of capturing images including one or more locators 114, or somecombination thereof. Additionally, imaging device 160 may include one ormore filters (e.g., for increasing signal to noise ratio). Imagingdevice 160 is configured to detect light emitted or reflected fromlocators 114 in a field of view of imaging device 160. In embodimentswhere locators 114 include passive elements (e.g., a retroreflector),imaging device 160 may include a light source that illuminates some orall of locators 114, which retro-reflect the light towards the lightsource in imaging device 160. Slow calibration data is communicated fromimaging device 160 to console 150, and imaging device 160 receives oneor more calibration parameters from console 150 to adjust one or moreimaging parameters (e.g., focal distance, focus, frame rate, ISO, sensortemperature, shutter speed, aperture, etc.).

I/O interface 170 is a device that allows a user to send action requeststo console 150. An action request is a request to perform a particularaction. For example, an action request may be to start or end anapplication or to perform a particular action within the application.I/O interface 170 may include one or more input devices. Example inputdevices include a keyboard, a mouse, a game controller, or any othersuitable device for receiving action requests and communicating thereceived action requests to console 150. An action request received byI/O interface 170 is communicated to console 150, which performs anaction corresponding to the action request. In some embodiments, I/Ointerface 170 may provide haptic feedback to the user in accordance withinstructions received from console 150. For example, haptic feedback isprovided by the I/O interface 170 when an action request is received, orconsole 150 communicates instructions to I/O interface 170 causing I/Ointerface 170 to generate haptic feedback when console 150 performs anaction.

Console 150 provides content to HMD 101 for presentation to the user inaccordance with information received from imaging device 160, HMD 101,or I/O interface 170. In the example shown in FIG. 1, console 150includes application store 152, tracking module 154, and engine 156.Some embodiments of console 150 have different or additional modulesthan those described in conjunction with FIG. 1. Similarly, thefunctions further described below may be distributed among components ofconsole 150 in a different manner than is described here.

Application store 152 stores one or more applications for execution byconsole 150. An application is a group of instructions, that whenexecuted by a processor, generates content for presentation to the user.Content generated by an application may be in response to inputsreceived from the user via movement of HMD 101 or interface device 170.Examples of applications include gaming applications, conferencingapplications, video playback application, or other suitableapplications.

Tracking module 154 calibrates system 100 using one or more calibrationparameters and may adjust one or more calibration parameters to reduceerror in determining position of HMD 101. For example, tracking module154 adjusts the focus of imaging device 160 to obtain a more accurateposition for observed locators 114 on HMD 101. Moreover, calibrationperformed by tracking module 154 also accounts for information receivedfrom IMU 116. Additionally, if tracking of HMD 101 is lost (e.g.,imaging device 160 loses line of sight of at least a threshold number oflocators 114), tracking module 154 re-calibrates some or all of thesystem components.

Additionally, tracking module 154 tracks the movement of HMD 101 usingslow calibration information from imaging device 160 and determinespositions of a reference point on HMD 101 using observed locators fromthe slow calibration information and a model of HMD 101. Tracking module154 also determines positions of the reference point on HMD 101 usingposition information from the fast calibration information from IMU 116on HMD 101. Additionally, tracking module 154 may use portions of thefast calibration information, the slow calibration information, or somecombination thereof, to predict a future location of HMD 101, which isprovided to engine 156.

Engine 156 executes applications within the system and receives positioninformation, acceleration information, velocity information, predictedfuture positions, or some combination thereof for HMD 101 from trackingmodule 154. Based on the received information, engine 156 determinescontent to provide to HMD 101 for presentation to the user, such as avirtual scene. For example, if the received information indicates thatthe user has looked to the left, engine 156 generates content for HMD101 that mirrors or tracks the user's movement in a virtual environment.Additionally, engine 156 performs an action within an applicationexecuting on console 150 in response to an action request received fromthe I/O interface 170 and provides feedback to the user that the actionwas performed. The provided feedback may be visual or audible feedbackvia HMD 101 or haptic feedback via I/O interface 170.

Flexure Guidance

A HMD, such as HMD 101, (or a near-eye display) uses a flexure basedguidance system to dynamically adjust a location of an image plane(e.g., to address vergence-accommodation conflict). The guidance systemis able to adjust a location of one or moveable elements (e.g.,electronic display 102 and/or one or more optical elements) of opticsblock 104 to control a location of an image plane. As used herein, amoveable element is an element whose movement corresponds to a change inlocation of the image plane. For example, in the case of a pancake lensassembly, the guidance system may be configured to move one or all ofthe optical elements of the pancake lens and/or the electronic display102 to change the location of the image plane.

The guidance system includes at least one actuator (e.g., a voice coilactuator, stepper motor, brushless DC electric motor, etc.) and aplurality of flexure elements that are able to flex and/or bend withmovement of the actuator to adjust an on-axis location of at least onemoveable element. The flexure elements can include, for example, aparallel beam flexure system (FIGS. 2-5B), a Roberts or dual Roberts(FIGS. 7-8B), diaphragm flexure, a straight line linkage based guidancewith a flexure based pivoting element, a wire based flexure, a beamarray, or some combination thereof.

Parallel Beam Flexure Guidance

FIG. 2 shows a diagram of a parallel beam flexure guidance system 200for use within HMD 101, in accordance with at least one embodiment.Parallel beam flexure guidance system 200 includes an actuator 202(e.g., a voice coil actuator, stepper motor, brushless DC motor, etc.),a first flexure element 204, and a second flexure element 206. The firstflexure element 204 comprises a first flexure beam 204 a and a secondflexure beam 204 b configured to guide movement of the electronicdisplay 102 along an optical axis 208 of an optics block (not pictured)and prevent tip/tilt in a first dimension (i.e., x-axis) and the secondflexure element 206 comprising a third flexure beam 206 a and a fourthflexure beam 206 b configured to guide movement of the electronicdisplay 102 along the optical axis 208 (or z-axis) and prevent tip/tiltin a second dimension (i.e., y-axis). Where the first flexure element204 and the second flexure element 206 guide movement of the electronicdisplay 102 along the optical axis 208 while limiting and preventtip/tilt and/or rotation of the electronic display 102 in the firstdimension (e.g., tip/tilt about the x-axis) and the second dimension(e.g., tip/tilt about the y-axis). In this embodiment, the varifocalactuation block 106 includes a combination of actuator 202 and parallelbeam flexure guidance system 200.

FIGS. 3-4 show additional views of a parallel beam flexure guidancesystem 200, in accordance with various embodiments, that include opticsblock 104. As shown, actuator 202 is positioned off-axis relative tooptics block 104, in one embodiment, and the first flexure element 204and the second flexure element 206 are arched in shape to conform to ashape around the optics block 104 within the HMD 101 to minimize theform factor of the HMD 101. At one end, the actuator 202 is fixedrelative to electronic display 102 in order to move electronic display102. At the other end, actuator 202 is fixed relative to the opticsblock 104. Accordingly, the first flexure element 204 extends from theactuator 202 curved or arched horizontally to a location 210 a that isfixed relative to the optics block 104. Similarly, the second flexureelement 206 extends from the actuator 202 curved vertically to alocation 210 b that is also fixed relative to the optics block 104.Thus, when actuator 202 is operated, the end of the actuator 202 fixedrelative to the electronic display 102 is extended to move theelectronic display relative to the optics block 104 and, thereby,adjusting the location of the image plane. This movement is discussed inmore detail below with respect to FIGS. 5A and 5B.

FIGS. 5A and 5B show a side view of the parallel beam flexure guidancesystem 200 of a HMD in operation, in accordance with variousembodiments. FIG. 5A shows the parallel beam flexure guidance system 200in a first state corresponding to a first location of the image plane.Here, the actuator 202 and guidance system 200 are in a default position(e.g., the first flexure element 204 and the second flexure element 206are not undergoing any flex or bend). The actuator 202 includes a firstend 502 and a second end 504 and, in operation, the actuator 202 causesthe distance between the first end 502 and the second end 504 to change,thereby, moving the image plane to different locations.

As shown in FIGS. 5A and 5B, the second end 504 of actuator 202 is acomponent made of molded plastic that attaches to the electronic display102, the first flexure element 204, and the second flexure element 206.Moreover, as shown in FIGS. 5A and 5B, the third flexure beam 206 a andthe fourth flexure beam 206 b of the second flexure element 206 arefixed to the molded plastic component at the second end 504 of actuator202 and another end of the third flexure beam 206 a and the fourthflexure beam 206 b are fixed to bracket 506 that is fixed relative tooptics block 104 (and which, in an embodiment, is integrated in a moldedplastic housing or pod of optics block 104).

Accordingly, when the distance between the first end 502 and the secondend 504 of the actuator 202 is increased to move the image plane to anew location, the first flexure element 204 and the second flexureelement 206 bend as the electronic display 102 is moved away from opticsblock 104, as shown in FIG. 5B. Thus, the parallel beam flexure guidancesystem 200 produces linear motion along the optical axis 208 whileminimizing parasitic error motions that includes straightness of travelof electronic display 102 relative to the optical axis 208 and angularerror. Moreover, the parallel beam flexure guidance system 200 producesthis motion within a form factor while minimizing power requirements.

FIG. 6 shows a diagram of a compound parallel beam flexure guidancesystem, in accordance with another embodiment. In this embodiment, theparallel beam flexure guidance system 200 includes an additional secondset of nested parallel beams (604 a and 604 b) between the first flexurebeam 204 a and the second flexure beam 204 b. In this example, theadditional second set of nested parallel beams (604 a and 604 b) operateto counteract off-axis translation that results from a projected beamlength foreshortening (of the first flexure beam 204 a and the secondflexure beam 204 b) that occurs during displacement. For example, ifelectronic display 102 translates in x and y (e.g., on the order of 30to 60 microns) in the single parallel beam example described above asbeams 204 a and 204 b bend while maintaining acceptable parallelism withrespect to optics block 104 (0.020 degrees of tip/tilt at extent oftravel), a compound parallel beam flexure guidance system, as shown inFIG. 6, can reduce the tip/tilt to close to zero. While FIG. 6 onlyshows a single compound parallel beam, it should be understood that acompound parallel beam guidance system would include a first compoundparallel beam to aid in prevent tip/tilt in a first dimension and asecond compound parallel beam to aid in prevent tip/tilt in a seconddimension consistent with the embodiments discussed with respect toFIGS. 2-5.

Roberts Flexure Guidance

FIG. 7 shows a diagram of a Roberts flexure guidance system 700 for usewithin HMD 101, in accordance with at least one embodiment. A RobertsMechanism is a linkage system that converts rotational motion toapproximate straight-line motion and flexures can be incorporated intosuch a mechanism and used as a guidance system that guides the motion ofelectronic display 102 along the optical axis of optics block 104 tominimize tip/tilt. Roberts flexure guidance system 700 include threeflexure elements (702 a, 702 b, 702 c), a pivot point 704 where each ofthe flexure elements are connected, a first anchor 706 fixed relative tothe optics block 104 which connects to flexure element 702 c, and a pairof second anchors (708 a and 708 b) that each connect to one of flexure702 a and flexure 702 b and that are fixed relative to electronicdisplay 102 and the moving end of actuator 202 via platform 710. Forclarification, platform 710 is fixed to the moving end of actuator andthe pair of second anchors (708 a and 708 b) are fixed to platform 710.Platform 710, in some embodiments, is optional, however, given thegeometry of a particular system, may become necessary, as will bediscussed further below. Thus, in operation, the Roberts flexureguidance system 700 provides substantially linear and repeatable motionbetween first anchor 706 and the pair of second anchors (708 a and 708b). This movement is discussed in more detail below with respect toFIGS. 8A and 8B.

FIGS. 8A and 8B show a side view of the Roberts flexure guidance system700 of a HMD in operation, in accordance with various embodiments. FIG.5A shows the Roberts flexure guidance system 700 in a first statecorresponding to a first location of the image plane. First anchor 706is fixed, in this embodiment, to optics block 104. While FIGS. 8A and 8Bshow platform 710 and second anchor 708 b as if these components areattached to the first end 502, the platform 710 is fixed to the movingor second end 504 of actuator 202 and a portion of the platform 710 thatincludes second anchor 708 b is merely hanging above and over the firstend 502 of actuator 202 in this embodiment. This becomes more apparentin FIG. 8B where the second end 504 of actuator 202 has moved along withplatform 710 while the first end 502 remains fixed. As shown in FIG. 8A,the actuator 202 and guidance system 200 are in a default position(e.g., the three flexure elements (702 a, 702 b, 702 c) are notundergoing any flex or bend).

As similarly discussed above, the second end 504 of actuator 202 is acomponent made of molded plastic that attaches to the electronic display102 and platform 710 of the Roberts flexure guidance system 700.Accordingly, when the distance between the first end 502 and the secondend 504 of the actuator 202 is increased to move the image plane to anew location, one or more of the flexure elements (702 a, 702 b, 702 c)bend as the electronic display 102 is moved away from optics block 104,as shown in FIG. 8B. As shown, the three flexure elements (702 a, 702 b,702 c) have been flexed about pivot point 704 causing flexure element702 b to move closer to flexure element 702 c while moving the platform710 in the z-direction relative to first anchor 706. Thus, the Robertsflexure guidance system 700 produces linear motion along the opticalaxis in a predictable and repeatable manner.

While FIGS. 7-8B only shows a single Roberts flexure, it should beunderstood that the Roberts flexure guidance system 700 includes a firstRoberts flexure to aid in prevent tip/tilt in a first dimension and asecond Roberts flexure to aid in prevent tip/tilt in a second dimensionconsistent with the embodiments discussed with respect to FIGS. 2-5.Moreover, as discussed with respect to FIG. 6, the Roberts flexureguidance system 700 may additionally include a compound Roberts flexureguidance system that include four total Roberts flexures (two for thefirst dimension and two for the second dimension).

Additional Flexure Guidance Considerations

In accordance with various embodiments, the flexure elements may besingle, compound, or some combination thereof. An example combinationmay include a straight line linkage based guidance system with a flexurebased pivoting element, such as a Sarrus linkage, Scissor linkage,straight-line linkage, parallel four bar, and so on. In one embodiment,the first flexure element and the second flexure element are pivots of aSarrus, Scissor, or straight-line linkage. The flexure elements can bemade from a metal (e.g., Titanium, stainless steel, spring steel,Beryllium Copper) and/or plastic (e.g., DELRIN, Polypropylene, etc.). Insome embodiments, the guidance system is designed to have what isreferred to as “infinite life,” or the property whereby the flexures ofthe guidance system operate below a strain or fatigue threshold of thematerial, above which the performance of the material of flexure beginsto degrade over time with use.

The guidance systems have a small form factor in order to facilitate usewith a HMD and/or a near-eye display. For example, as shown anddescribed further below, the flexure elements may be curved or arched tofurther decrease the form factor of the guidance system and, as aresult, the HMD. And in some embodiments, the flexure elements may beplastic that is formed (e.g., injected molded) to further decrease formfactor. One improvement in form factor is a decrease in a width of theflexures. Decreasing the width of the flexures can proportionallydecrease on-axis stiffness further but can result in a significantdecrease in off-axis stiffness (proportional to b3). To counter thisloss of off-axis stiffness, an additional flexure pairs can be added(e.g., from 4 to 6 flexures) to form a beam array. The result is a morecompact flexure guidance system with truer motion and minimal powerrequirements.

Flexure based guidance systems introduce very low amounts of tip/tilt(e.g., may be as low as 0.001 degree) during on-axis movement relativeto conventional systems. Moreover, the guidance systems offer a highfidelity of control over movement of the one or more moveable elements.This is important for optical systems, such as the pancake lensassembly, where a small amount of movement of a single optical elementcan correspond to a relatively large change in the image plane location.Accordingly, such guidance systems also offer a friction free systemthat provides smooth, continuous motion, with predictable (e.g.,deterministic, repeatability, etc.) motion behavior, that if notdesigned to fully cancel out off-axis translation, can still becalibrated out due to the repeatable and deterministic nature of themotion. For example, some of the guidance systems introduce someoff-axis translational displacement (e.g., in x and/or y) in themoveable element (e.g., the display) as it displaces the moveableelement on-axis (e.g., along z-axis). One advantage of the guidancesystems is that the translational off-axis displacement is deterministic(i.e., it occurs in a repeatable, consistent manner such that it can becalibrated out of the system). In some embodiments, the electronicdisplay 102 is calibrated to emit light in manner that offsets errorintroduced by the off-axis translational displacement of the moveableelement. Moreover, the compound configurations may also be used tomitigate translational movement using additional sets of flexureelements.

Additionally, since some actuators do not have passive position holding(i.e., no power), the flexure based guidance system may include apower-down, position hold or lock given that the flexure systems arespring biased toward a rested position where the flexures are in anunbent state. For example, the flexure based guidance system may includea mechanism, such as a disengaging pin, swing over paddle, or othermotion blocking mechanism, that effectively locks the electronic display102 in place when the HMD 101 is powered down and that unlocks theelectronic display 102 when the HMD 101 is powered up. The motionblocking mechanism may be actuated using a small stepper and leadscrew,bi-stable solenoid, nitinol wire, and so forth.

Moreover, a flexure based guidance system can be designed to mitigateadditional loading conditions that can result from orientation changeswith respect to gravity and from user head whips, for example. In someembodiments, the guidance systems may additionally include dampeners tomitigate overshoot caused by rapid and/or sudden adjustments in themoveable element.

Additional Configuration Information

The foregoing description of the embodiments of the disclosure has beenpresented for the purpose of illustration; it is not intended to beexhaustive or to limit the disclosure to the precise forms disclosed.Persons skilled in the relevant art can appreciate that manymodifications and variations are possible in light of the abovedisclosure.

Some portions of this description describe the embodiments of thedisclosure in terms of algorithms and symbolic representations ofoperations on information. These algorithmic descriptions andrepresentations are commonly used by those skilled in the dataprocessing arts to convey the substance of their work effectively toothers skilled in the art. These operations, while describedfunctionally, computationally, or logically, are understood to beimplemented by computer programs or equivalent electrical circuits,microcode, or the like. Furthermore, it has also proven convenient attimes, to refer to these arrangements of operations as modules, withoutloss of generality. The described operations and their associatedmodules may be embodied in software, firmware, hardware, or anycombinations thereof.

Any of the steps, operations, or processes described herein may beperformed or implemented with one or more hardware or software modules,alone or in combination with other devices. In one embodiment, asoftware module is implemented with a computer program productcomprising a computer-readable medium containing computer program code,which can be executed by a computer processor for performing any or allof the steps, operations, or processes described.

Embodiments of the disclosure may also relate to an apparatus forperforming the operations herein. This apparatus may be speciallyconstructed for the required purposes, and/or it may comprise ageneral-purpose computing device selectively activated or reconfiguredby a computer program stored in the computer. Such a computer programmay be stored in a non-transitory, tangible computer readable storagemedium, or any type of media suitable for storing electronicinstructions, which may be coupled to a computer system bus.Furthermore, any computing systems referred to in the specification mayinclude a single processor or may be architectures employing multipleprocessor designs for increased computing capability.

Embodiments of the disclosure may also relate to a product that isproduced by a computing process described herein. Such a product maycomprise information resulting from a computing process, where theinformation is stored on a non-transitory, tangible computer readablestorage medium and may include any embodiment of a computer programproduct or other data combination described herein.

Finally, the language used in the specification has been principallyselected for readability and instructional purposes, and it may not havebeen selected to delineate or circumscribe the inventive subject matter.It is therefore intended that the scope of the disclosure be limited notby this detailed description, but rather by any claims that issue on anapplication based hereon. Accordingly, the disclosure of the embodimentsis intended to be illustrative, but not limiting, of the scope of thedisclosure, which is set forth in the following claims.

What is claimed is:
 1. A headset comprising: an electronic display; anoptics block; a varifocal actuation system comprising: an actuatorpositioned off-axis relative to an optical axis of the optics blockconfigured to adjust a location of an image plane of the headset byvarying a distance between the electronic display and the optics block,the actuator having a movable end fixed relative to the electronicdisplay and a fixed end fixed relative to the optics block; and aguidance system comprising: a first flexure element guiding movement ofthe electronic display along the optical axis in a first dimension; anda second flexure element guiding movement of the electronic displayalong the optical axis in a second dimension, a first end of each of thefirst flexure element and the second flexure element being fixed to themovable end of the actuator and a second end being fixed relative to theoptics block.
 2. The headset of claim 1, wherein the first flexureelement and the second flexure element each include a first flexure beamand a second flexure beam, the first flexure beam of the first flexureelement and the second flexure element positioned parallel to the secondflexure beam.
 3. The headset of claim 2, wherein the first flexure beamand the second beam are arched in shape to conform to a shape around theoptics block within the headset.
 4. The headset of claim 2, wherein thefirst flexure element and the second flexure element are compoundparallel beam flexures that each include a third flexure beam and afourth flexure beam that are parallel to the first flexure beam and thesecond flexure beam.
 5. The headset of claim 1, wherein the firstflexure element is configured to minimize tilt of the electronic displayin the first dimension and the second flexure element guide isconfigured to minimize tilt in the second dimension.
 6. The headset ofclaim 1, wherein the first flexure element and the second flexureelement are Roberts flexure mechanisms, wherein the first end of eachRoberts flexure mechanism is a point of the Roberts flexure mechanismassociated with linear movement.
 7. The headset of claim 1, wherein thefirst flexure element and the second flexure element are pivots of aSarrus, Scissor, or straight-line linkage.
 8. The headset of claim 1,wherein the varifocal actuation system varies a distance between theelectronic display and the optics block along a z-axis, the firstdimension corresponds to an x-axis, and the second dimension correspondsto an y-axis, wherein the first flexure element guides movement of theelectronic display along the z-axis while minimizing tilt about thex-axis and the second flexure element guides movement of the electronicdisplay along the z-axis while minimizing tilt about the y-axis.
 9. Theheadset of claim 1, wherein the actuator is a voice coil actuator,stepper motor, or a brushless DC electric motor.
 10. The headset ofclaim 1, wherein the headset is a head mounted display (HMD) or a neareye display (NED).
 11. The headset of claim 1, further comprising: anencoder in communication with the varifocal actuation system configuredto: receive an input corresponding to a focal length for a frame of avirtual scene being presented on the electronic display; determine aposition for the electronic display relative to the optics block toachieve the focal length; and provide the actuator with an instructioncausing the electronic display to move relative to the optics block toachieve the focal length for the frame of the virtual scene.
 12. Avarifocal actuation system of a headset comprising: an actuatorconfigured to adjust an image plane at an exit pupil of the headset byvarying a distance between an electronic display and an optics block ofthe headset, the actuator having a movable end fixed relative to theelectronic display and a fixed end fixed relative to the optics block;and a guidance system comprising: a first flexure element guidingmovement of the electronic display in a first dimension; and a secondflexure element guiding movement of the electronic display in a seconddimension, a first end of each of the first flexure element and thesecond flexure element being fixed to the movable end of the actuatorand a second end being fixed relative to the optics block.
 13. Thevarifocal actuation system of claim 12, wherein the actuator ispositioned off-axis relative to an optical axis of the optics block. 14.The varifocal actuation system of claim 12, wherein the first flexureelement and the second flexure element each include a first flexure beamand a second flexure beam, the first flexure beam of the first flexureelement and the second flexure element positioned parallel to the secondflexure beam.
 15. The varifocal actuation system of claim 14, whereinthe first flexure beam and the second beam are arched in shape toconform to a shape around the optics block within the headset.
 16. Thevarifocal actuation system of claim 14, wherein the first flexureelement and the second flexure element are compound parallel beamflexures that each include a third flexure beam and a fourth flexurebeam that are parallel to the first flexure beam and the second flexurebeam.
 17. The varifocal actuation system of claim 12, wherein the firstflexure element is configured to minimize tilt of the electronic displayin the first dimension and the second flexure element guide isconfigured to minimize tilt in the second dimension.
 18. The varifocalactuation system of claim 12, wherein the first flexure element and thesecond flexure element are Roberts flexure mechanisms, wherein the firstend of each Roberts flexure mechanism is a point of the Roberts flexuremechanism associated with linear movement.
 19. The varifocal actuationsystem of claim 12, wherein the actuator is a voice coil actuator,stepper motor, or a brushless DC electric motor, and wherein thevarifocal actuation system is part of a head mounted display (HMD) or anear eye display (NED).
 20. The varifocal actuation system of claim 12,wherein the varifocal actuation system varies the distance between theelectronic display and the optics block along a z-axis, the firstdimension corresponds to an x-axis, and the second dimension correspondsto an y-axis, wherein the first flexure element guides movement of theelectronic display along the z-axis while minimizing tilt about thex-axis and the second flexure element guides movement of the electronicdisplay along the z-axis while minimizing tilt about the y-axis.